First author

Amphibians emerged from Earth's waters around 360 million years ago, evolving into terrestrial vertebrates — first reptiles, then birds and mammals. Since that time, land vertebrates have returned to the sea many times. Aquatic amniotes today — whales, manatees and sea snakes — bear live young whose sex is determined by chromosomes, or genotype, rather than by temperature, as for many egg-laying reptiles. Work by Chris Organ, an evolutionary biologist at Harvard University, and his colleagues, now suggests that Mesozoic marine reptiles also relied on genotypic sex determination (see page 389). Organ tells Nature more about the significance of these findings.

Was there evidence for genotypic sex determination in ancient marine reptiles?

No. The evolution of genotypic sex determination was poorly understood in extinct species, primarily because genetic and genomic characters don't fossilize.

What made this study possible?

First, recent palaeontological discoveries have revealed that mosasaurs and sauropterygians gave live birth — something that was already known in ichthyosaurs. Second, with Mark Pagel and Andrew Meade at the University of Reading, UK, we developed computational tools to make evolutionarily informed predictions about extinct species. Then, in collaboration with Daniel Janes at Harvard, we were able to think about the evolution of sex determination in new, interdisciplinary ways.

How did you apply these methods?

In the 1990s, Mark built a general framework to model correlated evolution among traits, each of which occur in two states, such as presence or absence. We extended this framework to predict the state of a character that did not fossilize in extinct species based on the rates at which the characters evolve in relation to one another, the state of the correlate that did fossilize and where the extinct species sit on the evolutionary tree.

What did you find?

We found that the evolution of live birth is dependent on the prior evolution of chromosomal sex determination and that extinct marine reptiles probably used chromosomes to determine the sex of their offspring. It's a nice demonstration of how characters change in concert with one another and of how the evolution of genomic characters can affect the success of species in different environments. It also shows that the fossil record continues to have a central role for understanding how traits evolve — even for traits that don't fossilize.